The technology of document scanning relates to a method and an apparatus whereby the visible image(s) carried by a document is converted to an electronic image signal, for use by a variety of electronic processing means.
In accordance with well known techniques, the scanning of a document operates to divide the document into an X-Y matrix comprising a large number of individual picture elements (PEL or PELS) equal in number to X times Y. The scanner apparatus generates an electronic image signal for each document PEL.
The document's visual image is converted into an image signal by the scanning process. This image signal can be used in many ways. For example, the image signal can be transmitted to remote locations for storage or for reproduction by a printer, or the image signal can be manipulated in a manner to enhance or change the image, or the image signal can be merged with other image signals.
Very simplistically, if a portion of the document's black image (assuming the document comprises a black image on white paper background) is detected within a document PEL, the binary signal "0" can be generated, in which case, the absence of a portion of the document's black image at a PEL location (i.e. the PEL contains only the white document background) results in a binary "1" being generated.
Usually, however, the image signal that is generated for each document PEL is a multi-bit digital signal. This is desirable since a multi-bit signal allows the sensing of different levels of blackness within each PEL area of the document (i.e. gray scale PEL information can be provided). Thus, a 4-bit signal per PEL allows 16 different color-shades of document image (i.e. white, 14 different shades of gray, and black) to be generated for each document PEL. The range of values of such a 4-bit signal would vary from a low value of hexadecimal "0", to a high value of "F". When an 8-bit per PEL signal is generated, the 256 different signals represent 256 different color-shades of document image for each document PEL. The range of values of such an 8-bit signal would vary from a low hexadecimal value of "00" to a high value of "FF".
A preferred embodiment of the present invention provides a document scanner that generates an 8-bit signal for each document PEL. The scan word "00" represents a black PEL. The scan word "FF" represents a white PEL, and intermediate value words represent shades of gray PELS. However, as those skilled in the art will readily appreciate, the invention is of general utility and is not to be limited to this specific multi-bit scan signal format.
Document scanners are conventionally provided with a generally horizontal, flat, glass platen on whose upper surface a document is placed, image side facing down, for scanning. The document can then be scanned by moving a line of light, or a linear footprint or zone of light, relative thereto, or alternatively the document can be scanned by moving the document relative to a stationary light footprint. The present invention will be described with reference to a moving-light document scanner. However it is to be understood that the invention is not to be limited to this particular type of scanner.
The convention used in the following description considers the direction in which the line of light extends to be the X direction. This is the direction in which the document PEL rows extend. The orthogonal scan direction is the Y direction. This is the direction in which the document PEL columns extend.
Each incremental scan position of the light footprint relative to the document defines a document row. While the scan operation usually comprises continuous motion, within the invention it need not be so, and in any event, the sequential reading out of the document sensor array has the effect of converting the document into a plurality of discrete rows. As will be appreciated, the position of the individual document columns is determined by the physical location of the individual sensor cells within the sensor array.
The prior art recognizes the problems that are created in document scanners by a variation in the amount of light that is emitted by the scanning light source. While various means, such as careful selection of the type of light source to be used, frequent replacement of the light source, and delaying the beginning of document scanning until the light source has reached full operation temperature, have been used in the art, all of these expedients are costly and/or prevent immediate use of the document scanner. For example, conventional light sources used in scanner devices require ten seconds or more after initial energization for a stabile light output to be reached.
Thus, the need exists for a scanner method and apparatus that will compensate for illumination changes in the scanning light source in a dynamic manner, for example during each and every document scan, to thereby allow document scanning to begin within a second or two after the light source has been initially energized.